According to the rules from the International Union of Pure and Applied Chemistry (IUPAC), the lanthanides (Ln) correspond to the series of chemical elements running from cerium (Z=58) to lutetium (Z=71).
Generally speaking, lanthanides form their most stable compounds when they are in the +3 oxidation state. The electronic structure of LnIII ions is that of xenon for LaIII, and then corresponds to the filling of the 4f orbitals up to [Xe] 4f14 for LuIII.
Lanthanides are known in the literature for their luminescent properties, which may be exploited in the context of numerous applications in the fields of photonics, optoelectronics and biology (magnetic and optical imaging and labelling).
At present, the majority of studies carried out with lanthanide complexes have been aimed at establishing luminescent probes containing long-lived emitters of visible light, especially EuIII and ThIII, or emitters of the near-infrared spectrum, such as PrIII, ErIII, YbIII or NdIII.
Lanthanides are particularly advantageous for applications in photonics and electronics on account of their unique emission properties. This is because they exhibit very narrow emission bands, providing high purity to the colour emitted. Moreover, the life of the excited states is particularly long, and the quantum yields of luminescence are high. Furthermore, the emission ranges may be adapted, and it is possible to obtain lanthanide complexes which emit within the wavelength range from ultraviolet (UV) to near-infrared (IR).
Since the 4f-4f transitions are forbidden in accordance with the rules of Laporte, the absorption coefficient of the lanthanides is very low, and their direct excitation requires the use of high-energy laser sources.
In order to allow emission of the metal with lower energies, it is necessary to sensitize the lanthanide by complexing it with a suitable organic chromophore, which is generally a conjugated system with high absorption in the UV-visible range, such as a diketone, which is capable of absorbing photons and transferring them efficiently to the lanthanide ions. The photons which are absorbed will excite the molecule and cause it to pass into a singlet state, which is able to relax, to return to the ground state, or else to pass into a triplet state, by inter-system conversion. If the triplet state of the metal is lower, energetically speaking, than that of the chromophore, there is then a transfer of energy (by Forster or Dexter mechanism) from the triplet state of the chromophore to the triplet state of the metal, which returns to the ground state and, in so doing, emits light.
For a system of this kind to operate efficiently, in other words to have a quantum yield of luminescence that is of advantage, it is necessary for the chromophore to absorb photons and transfer them efficiently to the metal by virtue of a high compatibility of its triplet state with that of the metal.
A number of different architectures have been proposed to date for the sensitization of lanthanide ions.
In the majority of cases, the chromophores comprise pyridine, bipyridine or terpyridine units which are capable of sensitizing the emission of lanthanides which emit in the visible range. These units have been functionalized with carboxylic acid groups in the optical field, to form stable complexes with lanthanides (Latva et al., Journal of Luminescence 1997, 75, 149-169 [1]). Advantageous quantum yields have been obtained for the complexes formed in this way. However, their stability remains low.
It has been shown, moreover, by Chatterton et al. (Angewandte Chemie, International Edition in English 2005, 44, 7595-7598 [2]) and by Nonat et al. (Chemistry, a European Journal 2006, 12, 7133-7150 [3]) that the introduction of three picolinate groups into tripodal architectures, or of four picolinate groups into a tetrapodal architecture, is able to produce lanthanide complexes which are stable and rigid, with quantum yields that are advantageous for terbium (45-60%) but markedly lower for europium (4-7%).
2-Hydroxyisophthalamide derivatives included in tetrapodal architectures have also been found to be effective in sensitizing terbium, leading to high quantum yields for this lanthanide (50-60%), but low quantum yields, on the other hand, for europium (6%) (Petoud et al., Journal of the American Chemical Society 2003, 125, 13324-13325 [4]), whereas satisfactory quantum yields (of the order of 44%) have been obtained for europium by the introduction of 2-hydroxypyridones into dipodal architectures (Moore et al., Inorganic Chemistry 2007, 46, 5468-5470 [5]).
Furthermore, the literature does not record quantum yields of more than 0.27% for neodymium, and even this value, published by Vögtle and his colleagues (ChemPhysChem 2001, 2, 769-773 [6]), is an exception, the quantum yields obtained for this lanthanide being typically 0.10%.
It would therefore be desirable to have compounds available which, as well as forming complexes with the lanthanides, are capable of efficiently sensitizing a number of these lanthanides, and particularly of sensitizing europium, terbium and neodymium.
The objective set by the Inventors, therefore, was to provide such compounds.
Another objective they set was that the syntheses of these compounds and lanthanide complexes should be easy to implement and should involve only reactions that are conventionally used in organic chemistry.
Another objective they set, further, was that these compounds should make it possible to obtain stable lanthanide complexes and, as far as possible, that this stability should exist both when the complexes are in solution and when they are in the solid state.
Another objective they set, still, was that the lanthanide complexes thus obtained should, when in the solid state, exhibit excitation wavelengths that are compatible with the substrates conventionally used in optics, such as glass substrates or indium tin oxide (ITO) substrates.
Another objective they set, finally, was that the said compounds should make it possible to eliminate the presence of molecules of water in the first coordination sphere of the lanthanides, at least when the complexes are in the solid state, in order to prevent or at least reduce the non-radiative relaxation phenomena associated with the oscillation of the OH bonds.